If you get off at Cannon Street mainline station and change to the Tube using Bank, only about 150 yards away, you’ll now only be charged the one fare instead of being slapped with two separate fares.
This makes life easier on Saturdays when Cannon Street Tube station is closed, and if you want to head west on the Central Line (using Cannon Street/Bank is easier than Charing Cross in my experience – just walk out of Cannon Street station, straight ahead up Walbrook and there’s Bank).
However, as Mike points out in the comments below, and amending a mistake of mine, this change doesn’t involve people who have to use Mansion House station on Saturdays when Cannon Street Tube is closed – a classic lack of co-ordination between Southeastern and Transport for London.
With most Greenwich line trains now running to Cannon Street rather than Charing Cross, hopefully it’ll help people use some of the advantages of the City terminal (other than being not normally stuck in a queue to enter London Bridge for five minutes).
These “out of station interchanges” aren’t publicised – most of them are straightforward local shortcuts that only regular users generally need to know about, like Catford to Catford Bridge – but you’ll find a full list here.
Now, if only getting off at Deptford and walking down Douglas Way to catch a London Overground train from New Cross (which takes about five minutes) would be charged as one fare too, my Oyster life would be (almost) complete.